Organochlorines such as DDT, its metabolite DDE, and polychlorinated biphenyls (PCBs) are persistent organic pollutants that may be toxic at levels encountered by the general population, through exposure in the food chain. PCBs interfere with neurodevelopment in animal experiments but human results are mixed. Each organochlorine has one or more types of biologic activity, and corresponding potential mechanisms of action. For example, a main concern about health effects of DDE has been because it interferes with androgen binding to its receptor. Most of my work in this area is based on analysis of serum from pregnant women who were in a cohort study named the Collaborative Perinatal Project (CPP). Here I describe some of my results from the CPP study. Results from other studies I have done on the health effects of persistent organic pollutants are indicated in the list of publications. Sera from over 50,000 pregnant women in the CPP have been frozen at -20 degrees Centigrade since 1959-1965, and with the health data on their offspring constitute a resource that is still actively used by many investigators. We selected a subset of about 2,800 children born in 1959-1966 and determined organochlorine concentrations in the corresponding stored maternal pregnancy sera, in 1997-1999. In earlier CPP papers we examined DDE levels in relation to fetal loss in previous pregnancies, and, in the CPP pregnancy, preterm birth, small-for-gestational-age, birthweight, and subsequent height;in all cases we found associations suggestive of adverse effects. To look for evidence of androgen-blocking, we examined maternal DDE level during pregnancy in relation to adjusted odds of cryptorchidism and hypospadias among their male offspring;for these outcomes the results were equivocal. Furthermore, we evaluated prenatal PCB exposure in relation to developmental assessment performance at 8 months of age, 7 years of age, and hearing as reflected by audiometric examination at age 8 years. None of these studies showed any associations. Last year's progress Because this project is now in the data analysis and reporting phase, the progress is reflected by presentations and publications. This year we published a paper on maternal pregancy serum organochlorine levels in relation to risk of birth defects in male offspring. We also published a paper examining whether levels of specific PCB congeners are informative regarding the activity of specific liver enzymes in humans. A manuscript on DDT and neurodevelopment is being prepared to submit to a journal.